Fluke 1623-2, 1625-2 Application Note

Taking care of

an at-risk dam

Application Note

Completed by the U.S. Army Corps of Engineers
(USACE) in 1962 as a flood-control measure, Tuttle
Creek Dam sits on the Big Blue River, five miles
north of Manhattan, Kansas. Made of rolled earth
and rock fill and resting on an alluvial foundation, it’s
about 137 feet high and 7500 feet long. The dam
holds back Tuttle Creek Lake, which amounts to
335,100 acre-ft at normal pool and approximately

1.9 million acre-ft during flood events.

Here’s the problem: It’s 12 miles
from the Humboldt fault zone, a
localized seismic “hot spot” that
has a small but real probabil­ity of producing an earthquake
of magnitude 5.7 to 6.6. Such
an event would cause what’s
known as liquefaction, in which
the earth (mostly silt and sand)
on which the dam rests changes
from a relatively solid base to
what amounts to quicksand.

During the 1989 Loma Prieta
earthquake in San Francisco,
the soil under the city’s Marina
district liquefied, causing many
buildings to collapse. A simi­lar event at Tuttle Creek could
cause the dam to fail. According
to 2006 USACE estimates, this
would release 381,000 cubic
feet of water per second, flood
parts of downtown Manhattan
to depths of 17 feet, result in the
deaths of up to 400 people out
of a population of 13,000, and
cause damages downstream of
$458 million. The dam has even
been featured on The History
Channel’s “Mega-Disasters: Dam
Break” show.

F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y

Bob Frazey uses the Fluke 1630 to check all the grounding wires around the building that all the communications come into from various points around and across
the dam. These include remote sensors, video feeds, and visual and audio warning stations.

In 2002 the Corps set out
to make the dam safe, using a
variety of methods, including
the construction of soil-cement
transverse panels to strengthen
foundation soils beneath the
toe of the dam. But it would
take years to finish that, and in
the meantime the area down­stream would still be at risk.
The answer was to put in place
a Dam Failure Warning System
that would sound an alert in
time for the people to evacuate.

The Corps of Engineers con­tracted for the Tuttle Creek Dam
Failure Warning System with the
global engineering, construc­tion and technical services firm
URS Corporation. With 55,000
employees worldwide, the
company has three divisions:
URS Engineering Corporation;
EG&G, a defense services com­pany; and Washington Division,
a large contracting company
and builder. URS Engineering
Corporation offers services to
rehabilitate and expand public
infrastructure, including sur­face, air, mass transit and rail
transportation networks, and
ports and harbors. The division
also provides program manage­ment; planning, design and
engineering; and construction
and construction-management
services for water supply, con­veyance and treatment systems.

The Dam Failure Warning
System is made up of a number
of components and subsystems,
starting with geotechnical
instruments.

Automated geotechnical
instruments

These devices include sensors to
measure seismic shaking, detect
embankment/foundation defor­mation, and monitor changes in
foundation pore pressures. The
data from these devices is sent
to the Critical Systems Building
(CSB) via radio, and many are
solar-powered, which not only
makes them immune to power
outages but also eliminates
possible voltage surges via ac
power lines.

Pore-pressure sensors

For pressure sensing, URS often
uses vibrating wire piezometers.
Housed in sturdy metal cases
with pointed ends, they are
pushed 30 to 50 feet into the
earth near the toe of the dam.
They are connected via cable
to a solar-powered datalogger
which in turn sends its data
to the central computer in the
CSB for storage and analysis.
The sensors’ output signal is a
frequency, which can be read
at fairly long distances despite
cabling losses. In addition, they
have built-in protection against

lightning surges, and when
coupled with good lightning pro­tection and grounding systems,
give very stable readings for
many years.

But they have a drawback,
says Jim Hummert, Jr., PE, Vice
President-Systems Engineer­ing with URS Corporation: It
takes about one second to get
a reading from each sensor.
While this is not a problem
for applications like long-term
performance monitoring for dam
safety (which generally involves
taking several readings over the
course of a day), it’s too slow for
an early warning system, which
must record a pore pressure
signature immediately following
an earthquake.

“We need to read these
devices more quickly and be
able to process the results
and run through some type of
algorithm or alarm-checking
protocol for notifications,” Hum­mert explains. For that reason
URS added a set of strain gage
pressure sensors with 4-20 mA
output. These can be read 10
to 15 times per second or more
with standard dam-safety data­acquisition equipment.

2 Fluke Corporation Taking care of an at-risk dam